Apparatus and method for performing initial cell search in wireless communication systems

ABSTRACT

The system and method of the present invention establishes a communication link between a user equipment (UE) and a base station in a communication system having a plurality of base stations which each transmit a common primary synchronization code (PSC) in a primary synchronization channel in conjunction with a base station specific secondary synchronization code (SSC) within a system frame, which receives with the UE an input signal including the PSC and SSC from at least one of the base stations. The UE analyzes the input signal to detect any received PSCs within a selected time period which has duration corresponding to the length of a system frame and determining a relative location of a strongest PSC within the selected time period. The input signal is then processed to remove the PSC from at least the determined PSC location. A secondary synchronization code (SSC) is then detected for the determined location from the processed signal. The communication link is then established using the detected SSCs.

BACKGROUND

[0001] The present invention relates to user equipment (UE)synchronization to a base station. More specifically, the presentinvention relates to an improved initial cell search method and system.

[0002]FIG. 1 illustrates a wireless communication system. Thecommunication system has a plurality of base stations 2 _(l-2) _(n) (2).Each base station 2 communicates with user equipments (UEs) 4 _(l)-4_(n) (4) within its operating area or cell 6 _(l)-6 _(n) (6).

[0003] When a UE 4 is first activated, it is unaware of its location andwhich base station 2 (or cell 6) to communicate. The process where theUE 4 determines the cell 4 to communicate with is referred to as “cellsearch.”

[0004] In typical code division multiple access (CDMA) communicationsystems, a multi-step process is used for cell search. For step one,each base station 2 transmits the same primary synchronization code(PSC) in a primary synchronization channel (PSCH). In a time divisionduplex (TDD) communication system using CDMA, the PSCH is one timeslotout of fifteen for case 1 cell search (as shown in FIG. 2a), such asslot 0 or in general K, or two timeslots for case 2 cell search (asshown in FIG. 2b), such as slots 0 or in general K and K+8 and 8. Eachbase station transmits the same PSC in the PSCH timeslot(s). To reduceinterference between secondary synchronization codes (SSCs) used in steptwo, each PSC is transmitted at a different time offsets. The PSCoffsets are at a set number of chips.

[0005] The UE 14 determines the base station 12 to be synchronized to bysearching the PSCH for received PSCs, such as using a matched filter. Anexample of the results of such a search are shown in FIG. 4. As shown inFIG. 4, peaks 26 ₁-26 ₂ occur in the PSCH where there is a highcorrelation with the PSC code. Typically, the search results areaccumulated over multiple frames to improve accuracy. Using theaccumulated results, the PSC peak locations are determined in the PSCH.

[0006] Referring back to FIG. 2a and 2 b, along with each base station'stransmitted PSC, each base station 12 also simultaneously transmitssecondary synchronization codes (SSCs), such as three, for both TDD case1 and case 2. The SSCs sent by each base station 14 are used to identifycertain cell parameters, such as the code group and frame timing used bythe cell. The UE 14 typically uses a correlator to detect the SSCs andthe data modulated on them at each PSC peak identified in step I. The UE14 to read the broadcast control channel. In TDD step III for both typesI and II, typically, the UE 14 detects the midamble used in thebroadcast channel and subsequently reads the broadcast channel.

[0007] A drawback of the initial cell search system described above isthat the performance of the second step (SSC detection) is governed bythe quality of the received signal which could result in falsedetections if this signal is of poor quality. In past systems, thesecond step, receives no benefit from successful execution of step 1.

[0008] Accordingly, there is a need for an initial cell search systemwherein the second step's performance is not solely governed by thereceived input signal, providing more accurate SSC detection.

SUMMARY

[0009] The system and method of the present invention establishes acommunication link between a user equipment (UE) and a base station in acommunication system having a plurality of base stations which eachtransmit a common primary synchronization code (PSC) in a primarysynchronization channel in conjunction with a base station specificsecondary synchronization code (SSC) within a system frame, whichreceives with the UE an input signal including the PSC and SSC from atleast one of the base stations. The UE analyzes the input signal todetect any received PSCs within a selected time period which hasduration corresponding to the length of a system frame and determining arelative location of a strongest PSC within the selected time period.The input signal is then processed to remove the PSC from at least thedetermined PSC location. A secondary synchronization code (SSC) is thendetected for the determined location from the processed signal. Thecommunication link is then established using the detected SSCs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is an illustration of a wireless communication system.

[0011]FIGS. 2a and 2 b are illustrations of the physical synchronizationchannel (PSCH) for case 1 and case 2, respectively.

[0012]FIG. 3 is an illustration of peaks in a PSCH.

[0013]FIG. 4 is a block diagram of the initial cell search system of thepresent invention.

[0014]FIG. 5 is an exemplary block diagram of a step 2 processor.

[0015]FIG. 6 is an exemplary block diagram of a step 3 processor.

[0016]FIG. 7 is a flow diagram of the initial cell search system of thepresent invention.

[0017]FIG. 8 is a block diagram of a second embodiment of the initialcell search system.

[0018]FIG. 9 is a block diagram of a third embodiment of the initialcell search system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The preferred embodiments will be described with reference to thedrawing figures where like numerals represent like elements throughout.

[0020] The initial cell search system 10 in accordance with thepreferred embodiment of the present invention is illustrated in FIG. 4.The system 10 comprises a step 1 processor 12, a cancellation device 18,a step 2 processor 14, and a step 3 processor 16, to accomplish initialsynchronization between a user equipment (UE) and a base station.

[0021] Step 1 of the initial cell search algorithm is accomplished usingthe step 1 processor 12. FIG. 4 shows one implementation of a step 1processor, although others may be used. The step 1 processor 12comprises a Hierarchical Golay Correlator (HGC) 21 and a PSC decisiondevice 22. The purpose of the step 1 processor 12 is to find thestrongest base station's PSC over a frame or multiple frames worth ofsamples. A chip sampled input signal I is received by the UE andprocessed by the HGC 21. The HGC 21 is a reduced complexityimplementation of the correlation process between PSC and the inputsignal I at consecutive chip locations. The output of the HGC 21represents the magnitudes of the detected PSC power levels for thosebase stations detected by the HGC 21. The base stations' PSCs with ahigh received power level appear as peaks in the frame. The outputs fromthe HGC 21 are output to the PSC decision device 22.

[0022] The PSC decision device 22, coupled to the HGC 21, receives thecorrelation values output by the HGC 21 for each chip in a frame worthof chips. A frame's worth of chips is preferably equivalent to thesystem frame, which by way of example, is equivalent to 38,400 chips. Asthose having skill in the art know, the system frame can be more or lessthan that which is used in this disclosure.

[0023] The decision device 22 stores each chip correlation value fromthe HGC 21 over a predetermined number of frames N and averages eachchip's correlation values. As an example, a system frame is 4 chipslong, and N=2. The HGC 21 outputs the correlation values A₁, B₁, C₁, andD₁, respectively for each of the four chips. The decision device 22stores these values and receives the output of the next frame'scorrelation values for each chip from the HGC 21, which are A₂, B₂, C₂,D₂. Each chip's correlation values are then averaged, (i.e., A₁,+A₂/2;B₁+B₂/2; C₁+C₂/2; D₁+D₂/2).

[0024] Once the decision device 22 finds the average correlation valuefor each average correlation chip in a frame, the position of themaximum average of the frames is determined and its value compared witha determined threshold. The threshold is based on the noise level (i.e.,interference plus thermal noise) at the receiver. The noise estimator 24has an auxiliary HGC (not shown) that is based on a code which has verylow cross correlation with the PSC and the SSCs. The noise estimator HGCcalculates a noise estimate for every chip in the system frame. Thenoise estimator iterates over the same number of frames as the HGC 21and averages several of the noise estimates in a window around theestimated PSCH location. The window size is preferably about 128, i.e.,64 chips on both sides of the PSCH location. As those having skill inthe art know, the window size may be larger or smaller than 128.

[0025] If the maximum average is greater than the threshold, thedecision device 22 determines whether the transmission pattern of thebase station associated with the maximum average location is case 1 orcase 2. This determination is made by comparing the correlation value ofthe chip at the maximum location +(8*2560) or maximum location+(7*2560). If this value is greater than the threshold, then thetransmission pattern is case 2. Otherwise, the transmission is case 1.

[0026] If the maximum location value is less than the threshold, thestep 1 processor 12 continues processing the input signal I until acorrelation value greater than the threshold is found or a failedcondition met. As those skilled in the art know, the decision processor22 may utilize any of a number of methods for determining the locationof the strongest PSC code. Once the maximum location is found, thedecision processor 22 forwards the location and the PSC to thecancellation device 18 and the step 2 processor 14.

[0027] The cancellation device 18, coupled to the step 1 processor 12and the step 2 processor 14, takes the maximum location, the PSC and theinput signal I and subtracts the PSC from the input signal I. Thissubtraction eliminates the PSC from the chip at the maximum location inthe input signal I. The subtraction of the PSC from the input signal Ican be done by one of several cancellation methods, such as interferencecancellation. Using interference cancellation, the PSC is converted,using an interference construction device (not shown), into an estimateof its contribution to the input signal I. The received PSC'scontribution is subtracted, such as by a subtractor. The resultingsignal has the PSC's contribution removed from the input signal I at themaximum location. In code multiplexing systems, one code appears asnoise to other codes. Accordingly, the PSC is essentially noise to theSSC. As a result, when the PSC is cancelled from the input signal I, thestep 2 processor 14 is able to locate the SSC and slot offset withgreater accuracy and speed.

[0028] The step 2 processor 14, coupled to the cancellation device 18,the step 1 processor 12 and the step 3 processor 16, receives themodified input signal from the cancellation device 18 and the locationof the PSC from the step 1 processor 22.

[0029] One example of a step 2 device is illustrated in FIG. 5, althoughothers may be used. This step 2 device comprises correlator 31, a fastHadamard transform device (FHT) 33, phase estimator device 37, aderotate device 34, an accumulator 36, and a decision device 39. Sincethe location of the PSC has been determined by the step 1 processor 12,then the step 2 processor 14 need only search for the SSCs in themaximum location input from the step 1 processor 12. In this step, theUE identifies the code group and the t_(offset) associated with the basestation at the maximum location. The step 2 processor 14 also determinesthe frame index number within the interleaving period of two frames andit determines the slot index (K or K+8). As those skilled in the artknow, the t_(offset) determined in this step allows the UE tosynchronize to the slot boundary. The modified input signal and theposition of the PSC are input to the correlator 31. The correlator 31,coupled to the FHT 33 and the cancellation device 18, correlates thereceived input signal with the length 256 chip code at the PSC positionto obtain 16 correlation values. This code, C_(R), is obtained from chipby chip multiplication of first SSC, C₁, and a masking sequence, Z. Thisis shown below:

C _(r)(i)=C _(l)(i)*z(i),i=0, . . . ,255  Equation 1

[0030] The 16 complex correlation values, R_(c) (K) are obtained usingthe above code. R_(c) (K) is obtained by the following equation 2:$\begin{matrix}{{{r_{c}\left( {k,n} \right)} = {\sum\limits_{i = 0}^{15}\quad {{c_{r}\left( {{16k} + i} \right)}{R_{x}\left( {{t_{cp} + {16k} + i},n} \right)}}}},{k = 0},\ldots \quad,{15;{n = 0}},\ldots \quad,N} & {{Equation}\quad 2}\end{matrix}$

[0031] Where t_(cp) is the PSC position obtained from the step 1processor 12 and N is the maximum number of PSCH time slots used foraveraging.

[0032] The correlation values obtained at the output of the correlator31 are applied to the FHT 33. The FHT 33 is coupled to the correlator 31and a derotate device 34, obtains 16 complex correlation values thatcorrespond to the correlation of 16 SSCs and the received signal. Thatis, $\begin{matrix}\begin{matrix}{{{r_{f}\left( {k,n} \right)} = \quad {{FHT}\left\{ r_{c{({k,n})}} \right\}}},} \\{{\approx \quad {\sum\limits_{i = 0}^{255}\quad {c_{{k{(i)}} \cdot {z{(i)}}} \cdot {R_{x}\left( {{t_{cp} + i},n} \right)}}}},{k = 0},\ldots \quad,{15;}} \\{\quad {{n = 0},\ldots \quad,N}}\end{matrix} & {{Equation}\quad 3}\end{matrix}$

[0033] As those skilled in the art know, taking FHT of R_(c)(K)'s isequivalent to the correlation of unmasked SSCs with the received signal.This is possible due to the special structure of the 16 SSCs. Pleasenote that a case 1 signal uses six (6) SSCs and a case 2 signal usestwelve (12) SSCs. Four (4) SSCs are unused.

[0034] The phase estimator 37 receives the modified chip sampledreceived signal, as well as the PSC position from the step 1 processor12. The output of the step 1 HGC 21 at the PSC position corresponds tothe correlation of the PSC with the received signal at the PSC position.This complex correlation value is the input to the phase estimator 37.In this phase estimator 37, the complex correlation value is normalizedand then conjugated. The phase estimation is necessary for thederotation of the SSCs.

[0035] The derotate device 34, coupled to the phase estimator 37 and theFHT 33, receives the 16 SSCs from the FHT 33 and the phase estimationfrom the phase estimator 37. The derotate device 34 derotates the outputof the FHT 33. The derotation phase is the phase of the PSC. The complexcorrelation values are complex multiplied with the phase.

[0036] The derotated correlation values are then forwarded to theaccumulator 36. The accumulator 36 is coupled to the derotate device 34and the step 2 decision device 39. The derotated correlation values areadded coherently with a period of two (for case 1) or four (for case 2),for N iterations in accordance with equation 4:

r _(a) ^(l)(k,n)=r _(a) ^(l)(k,n−1)+rd(k,n)δ(l−n mod L), k=0, . . . , K;n=0, . . . , N;l=0, . . . L  Equation 4

[0037] where N is the maximum number of iterations to obtain a reliablesignal value, K is the number SSCs used (K=6 for case 1 and K=12 forcase 2) and L is periodicity of PSCH (L=2 for case 1 and L=4 for case2). These correlation values are initially set to zero. The decisionvariables are formed from the correlation values according to the SSCtransmission patterns.

[0038] The decision variables obtained in the accumulator 36 areforwarded to the decision device 39. There are 64 decision variables forcase 1,32 code groups and 2 frames indices. For case 2, there are 128decision variables, 32 code groups, 2 frame indices and 2 slots (K orK+8). The decision device 39 compares all the decision variablessequentially (one by one). This scheme is efficient since the number ofdecision variables is not large and the scheme can be implementedwithout much complexity. The transmission pattern that the maximumdecision variable belongs to indicates the code group number of case 1and case 2 and PSCH slot index for case 2.

[0039] The t_(offset), scrambling code group number, SSCs, and thelocation of the PSC are then forwarded to the step 3 processor 16. Thestep 3 processor 16, coupled to the step 2 processor 14, retrieves themidambles and primary scrambling code that are used by the UE. The codegroup number retrieved by the step 2 processor 14 is associated withfour cell parameters. Therefore, identification of the code group numberidentifies the midamble codes used by the cell. The four cell parametersassociated with the code group are cycled through System Frame Numbers(SFNs) as depicted in Table 1. TABLE 1 Cell Parameter (initially CellParameter Cell Parameter Code Group assigned) (SFN mod 2 = 0) (SFN mod 2= 1) i = 1, . . . , 32 4(i − 1) 4(i − 1) 4(i − 1) + 1 4(i − 1) + 1 4(i− 1) + 1 4(i − 1) 4(i − 1) + 2 4(i − 1) + 2 4(i − 1) + 3 4(i − 1) + 34(i − 1) + 3 4(i − 1) + 2

[0040]FIG. 6 illustrates an exemplary step 3 processor 16. Although astep 3 processor is illustrated, any step 3 processor may be utilized.The step 3 processor 16 comprises a correlation device 41, anaccumulation device 42, and a decision device 43. The correlation device41 is forwarded to the code group and frame index from the step 2processor 14, and the PSC position from the step 1 processor 12. Aperiodic window size pWS and multipath window size mpWS are also inputto the correlation device 41. The input signal I is correlated with thefour (4) midambles that are associated with the code group by thecorrelation device 41. The correlation is performed at WS3 calculatedcandidate midamble locations on the P-CCPCH which are determined by thet_(offset) of the code group, the periodic window size pWS and themultipath window size mpWS; where WS3=pWS+2mpWS.

[0041] The basic midamble code toggles with the SFN (odd/even). If theSFN is even, the correlation device 41 correlates against the basicmidamble code. If the SFN is odd, the correlation device 41 correlatesagainst the cycled midamble code. For example, in the case of code group0, the correlation device 41 correlates against midamble codes 0,1,2 and3 on even SFN, and the correlation device 41 correlates against midamblecodes 1,0,3 and 2 on odd SFN. It should be noted that cell search doesnot know the SFN, but it does know whether the SFN is even or odd basedon the frame index (1 or 2) found by the step 2 processor 14.

[0042] The correlation device 41 calculates 4xWS3 correlations. Theperiodic window allows the correlation device 41 to find the maximumcorrelation. The purpose of the multipath window is to adjust the PSCHposition to include the maximum amount of multipath. This may benecessary if the strongest multipath component is not the firstsignificant multipath component.

[0043] The correlation values output from the correlation device 41, areforwarded to the accumulation device 42 which is coupled to thecorrelation device 41 and the decision device 43. The accumulationdevice 42 accumulates the correlation values over a predetermined numberof frames N3. It should be noted that initial cell search does not knowframe boundaries so the initial cell search system typically uses blocksof 38400 chips (2560 chips×15 slots) in lieu of frames. The accumulationdevice 42 forms the decision variables by adding the absolute value ofthe real and imaginary parts of the complex number that represents thecorrelation value. A decision variable is the magnitude measure of thecorresponding correlation value. In order to have a more reliabledecision, these decision variables can be accumulated for N3 iterations,where N3 is the maximum number of iterations for a reliable signal tonoise ratio level.

[0044] The decision variables generated by the accumulation device 42are forwarded to the decision device 43. The decision device 43, coupledto the accumulation device 42, determines the maximum decision variableby simple sequential comparison. The maximum decision variablecorresponds to the basic midamble used for the cell. The scrambling codenumber associated with the identified midamble is the scrambling code ofthe cell. The scrambling code is then utilized by the UE for broadcastchannel processing.

[0045] The flow diagram for the initial cell search system isillustrated in FIG. 7. The UE receives the input signal over the commondownlink channel (step 601). The step 1 processor 12 detects thelocation of the PSC associated with the strongest base station (step602). The step 1 processor 12 forwards the PSC to the cancellationdevice 18 (step 603). The cancellation device 18 then subtracts the PSCdetected from the step 1 processor 12 from the input signal I (step 604)and forwards this modified signal to the step 2 processor 14 (step 605).Using the modified input signal from the cancellation device 18 and thelocation of the PSC from the step 1 processor 12, the step 2 processor14 retrieves the SSCs and determines t_(offset) and the code groupnumber associated with the strongest base station (step 607). The codegroup number is then forwarded to the step 3 processor 16 (step 608)which retrieves the midambles and primary scrambling codes therefrom(step 609). These codes are then used by the UE to synchronize to thebase station (step 610).

[0046] Since the second step of the initial cell search is the weakest,the cancellation of the PSC from the signal input to the step 2processor 14 provides a cleaner signal and results in a betterestimation of the SSCs time. This results in a more accurate slot offsetand code group number determination. Ultimately, this procedure reducesthe number of false detections by the UE.

[0047] A second embodiment is illustrated in FIG. 8. Similar to thesystem of FIG. 1, the system of this second embodiment utilizes acancellation device 18 ₂ to subtract the PSC and SSCs from the inputsignal I before processing by the step 3 processor 16. Step 2 does notreceive a PSC removed input signal, instead the modified input signal tothe step 3 processor 16 is able to more accurately detect the midambleand code group of the detected base station.

[0048] A third embodiment is illustrated in FIG. 9. This thirdembodiment utilizes the cancellation devices 18 1 and 18 ₂ to improvethe accuracy of the initial cell search system 10. The cancellationdevice 18 ₁ removes the PSC from the detected location in the inputsignal prior to the step 2 processor 14. The cancellation device 18 ₂removes the SSCs prior to the step 3 processor 16.

What is claimed is:
 1. A method for establishing a communication link between a user equipment (UE) and a base station in a communication system having a plurality of base stations which each transmit a common primary synchronization code (PSC) in a primary synchronization channel in conjunction with a base station specific secondary synchronization code (SSC) within a system frame, the method comprising: receiving with the UE an input signal including the PSC and SSC from at least one of the base stations; analyzing said input signal to detect received PSCs within a selected time period frame and determining a relative location of a strongest PSC within system frame; and processing said input signal to remove the PSC from at least the determined PSC location, and detecting a secondary synchronization code at the determined location from the processed signal.
 2. The method of claim 1 further comprising the step of detecting a scrambling code number for determining cell parameters of a base station associated with said detected SSCs.
 3. The method of claim 2 wherein said removal of said detected PSC includes interference cancellation.
 4. The method of claim 1 further comprising the steps of: processing said input signal to remove the PSC and SSC from at least the determined PSC location; and detecting a scrambling code number from the processed signal for determining cell parameters of a base station associated with said detected SSC.
 5. The method of claim 4 wherein said removal of said detected PSC and said SSC includes interference cancellation.
 6. A communication system including a plurality of base stations which each transmit a common primary synchronization code (PSC) in a primary synchronization channel in conjunction with a base station specific secondary synchronization code (SSC) within a system frame, and a user equipment (UE) comprising a cell search system for establishing a communication link between a UE and a base station, the UE for receiving an input signal including the PSC and SSC from at least one of the base stations, said cell search system comprising: a first processor analyzing said input signal to detect received PSCs within a selected time period and determining a relative location of a strongest PSC within the system frame; a cancellation processor for processing said input signal to remove the PSC from at least the determined PSC location; and second processor for detecting said SSCs at the determined location from the processed signal.
 7. The system of claim 6 wherein said cell search system further comprises a third processor, responsive to said SSCs, for detecting a scrambling code number of the base station associated with said determined location.
 8. The system of claim 7 wherein said cancellation processor uses interference cancellation to remove said PSC from said input signal.
 9. A user equipment (UE) comprising a cell search system for establishing a communication link between the UE and a base station in a communication system having a plurality of base stations which each transmit a common primary synchronization code (PSC) in a primary synchronization channel in conjunction with a base station specific secondary synchronization code (SSC) at a different time within a system frame, said UE receiving an input signal including the PSC and SSC from at least one of the base stations; said cell search system comprising: a first processor analyzing said input signal to detect received PSCs within a selected time period which has a duration corresponding to the length of a system frame and determining a relative location of a strongest PSC within the selected time period; a cancellation processor for processing said input signal to remove the PSC from at least the determined PSC location; and second processor for detecting said SSCs for the determined location from the processed signal; said system using the detected SSCs to establish the communication link.
 10. The UE of claim 9 wherein said cell search system further comprises a third processor, responsive to said SSCs, for detecting a scrambling code number of the base station associated with said determined location.
 11. The UE of claim 10 wherein said cancellation processor uses interference cancellation to remove said PSC from said input signal.
 12. A method for establishing a communication link between a user equipment (UE) and a base station in a communication system having a plurality of base stations which each transmit a common primary synchronization code (PSC) in a primary synchronization channel in conjunction with a base station specific secondary synchronization code (SSC) within a system frame, the method comprising: receiving with the UE an input signal including the PSC and SSC from at least one of the base stations; analyzing said input signal to detect received PSCs within a selected time period frame and determining a relative location of a strongest PSC within system frame; detecting a secondary synchronization code at the determined location from said input signal; and processing said input signal to remove the PSC and SSC from at least the determined PSC location.
 13. The method of claim 12 further comprising the step of detecting a scrambling code number from the processed signal for determining cell parameters of a base station associated with said detected SSC.
 14. The method of claim 2 wherein said removal of said detected SSC includes interference cancellation. 